Electronic fuel controls of the type employed to meter fuel to gas turbine engines are well known in the art. These prior art fuel controls are designed to permit the operator/pilot to manually position the main fuel metering valve in the event of a failure in the automatic metering valve positioning system. For a discussion of such manual backup systems, and the potential problems associated therewith, reference may be had to U.S. Pat. No. 4,302,931.
As discussed in U.S. Pat. No. 4,302,931, the typical manual backup system for an electronic control consists of a direct mechanical link between the pilots control lever and the fuel metering valve. This direct mechanical connection can be potentially dangerous in that no protection against overfueling is provided and, accordingly, excessive engine temperatures and turbine speeds or flame-out can result from excessively quick action by the pilot. It has been proposed, referring again to U.S. Pat. No. 4,302,931, to employ a secondary flow limiting valve downstream of and in series with the main fuel metering valve and to provide a control for this secondary valve which will prevent the pilot from exceeding safe operating limits when in the manual mode. The controls for such secondary valves, however, add complexity to the fuel control. Further, and in any event, it is important that pilots be trained in the proper operation of the manual backup system and that the training be accomplished in a manner which insures against exceeding the safety limits of critical engine operating parameters. Such safe training is not possible using prior art controls and practices.